Invasive species cure worse than disease?

A long-term scientific study shows herbicides may cause more problems than they resolve for large-scale treatment of invasive species.

Aug 25, 2010

Invasive species cure worse than disease?

By ALAN NEWPORT

A long-term scientific study shows herbicides may cause more problems than they resolve for large-scale treatment of invasive species.

“Our study presents a cautionary tale about invasive-species management: The treatment can be worse than the disease,” says Matt Rinella, an ecologist at the Fort Keogh Agricultural Experiment Station in Miles City, Mont.

Rinella revisited a Montana study done in the early 1980s that treated leafy spurge with herbicide and measured the results. He and other researchers recently followed up on that study 16 years after the herbicide treatment.

In brief, Rinella says they found leafy spurge overcame the large-scale spraying and was as prevalent or worse 16 years later. In fact, the original data showed it recovered to near pre-spraying levels four years after the first treatment.

Rinella says it’s probable, though not certain, that leafy spurge made up a higher percentage of the biomass 16 years after the spraying. He explains that some statistical errors in counting biomass make it uncertain.

But what makes that likelihood more certain is other research that shows both grasses and native forbs compete for space and resources with leafy spurge, so the reduction of native forbs would leave more room for the invader.

Part of the downside to this story is some native forbs appear to have suffered serious long-term depletion, and perhaps irreparable damage, from the herbicide treatment. Two native wildflowers, Missouri goldenrod and yarrow, were nearly extinct in the sprayed plots. Without herbicide, these two species proved capable of coexisting indefinitely with the invasive species. Although the herbicide appeared to be fully dissipated within two years, the plant community may have been permanently altered.

This situation is a problem because elimination of native species apparently can create more opportunity for exotic species, and a decrease in biodiversity nearly always makes an ecosystem weaker.

Effects on the rest of the plant community varied:

• When herbicide wasn’t used, many native forbs did similarly well in grazed and non-grazed plots.

• Four native perennials became rarer in sprayed plots only when grazing was excluded.

• Plots that were sprayed and grazed fared better overall than plots that were sprayed and not grazed.

• The herbicide continued to suppress three native forbs one year after application, but like the exotics, these plants recovered by the study’s end.

• Grass production increased the year of spraying, and most likely the next year.

Rinella says these findings don’t mean invasive-species control with herbicides is universally a bad idea, but he thinks it shows large-scale treatment may be counterproductive.

Rinella and the other researchers suggest this study also could provide a broader warning for invasive-species management beyond the range-management community.

Plots show herbicide, grazing effects

Initially, this weed-control project studied the effectiveness of the herbicide Tordon on leafy spurge from 1982 to 1984.

In the project, 10,000 acres were sprayed — minus several areas covered with tarps for research purposes — at the N-Bar Ranch near Grass Range, Mont. Grazing exclosures also were erected on some of the plots to keep cattle off. This created repetitions of plots that were:

• sprayed and grazed

• sprayed and not grazed

• not sprayed and grazed

• not sprayed and not grazed

Sixteen years later, the leader of the project took Matt Rinella, who was then a graduate student, to visit the N-Bar Ranch with other students and faculty. The exclosures were still standing, which presented the opportunity to study how a one-time aerial spraying of herbicide and more than a decade of grazing, or lack of it, affects the rangeland.